As I was researching articles to review to post to the blog, I came across a title that caught my curiosity. "Quantifying The Collisionless Nature Of Dark Matter And Galaxies In A 1689";(Doron Lemze, Yoel Rephaeli, Rennan Barkana, Tom Broadhurst, Rick Wagner & Mike L. Norman). I found this while searching Worldwidescience.org on topics of surface profile of galaxies and densities.
The word "collisionless" caught my attention because when we think of collisions we think of two or more masses making contact and either combining into one mass or bouncing off each other into different directions from their original trajectory. So I quickly assumed that collisionless refers to the lack of contact between the masses indicated as dark matter.
The authors state that despite a recent collision of two massive clusters, the spatial distribution of dark matter and galaxies are quite similar. A conical shaped shock front is visible indicating two clusters have passed through each other with an obvious collisionally merged gas distribution, but the galaxies and the lensing mass are largely intact, implying straightforwardly that dark matter and galaxies are collisionless (Markevitch et al. 2002; clowe et al. 204; Bradac de al, 2006). It implied that once hydrostatic equilibrium is reached, dark matter and galaxies should have the same mean specific kinectic energy. The total specific kinectic energy of dark matter is proportional to that of the gas which was evident through X-ray and optical observations of groups and clusters using mean emission-weighted gas temperature scales.
The paper was very technical in nature and hard to follow as to their methodologies (probably due to my limited understanding of the processes used). It goes in detail in the use of calculated velocities of dark matter and galaxies as well as galaxies surface number densities and projected velocities dispersions.The paper is a continuing comprehensive study of the dynamical properties of dark matter, galaxies and the hydrodynamics in cluster A 1689. It assumes that if dark matter and galaxies are fully collisionless, they should have the same average specific kinectic energy.
The one thing that the paper does not do is exam the electromagnetic aspect of a "collisionless" particle. that is attraction versus repulsion forces. In my simple mind, I would expect the simple nature of chemistry and physics to come into play. If the gas particles are ionic in nature, would these tend to be more positive thus repulsive as they approach each other therefore, never actually colliding? I guess that is why I am taking this course turning my simple understanding into more highly complex confusion.
So, as with so many things in astrophysics and science in general, the difference between everyday uses of words and their scientific meaning muddies the issue. In this case its the word 'collision'.
ReplyDeleteIn everyday life, and down to the atomic level, a collision between two objects (e.g. a tennis ball against a racket, your hand against a table, two hydrogen atoms against each other) is an electromagnetic interaction between the two objects - mostly a repulsive interaction between the electrons in atoms.
In physics, collisions generally refer to interactions between particles or compound objects due to any of the three fundamental microscopic interactions (electromagnetic, weak, strong). Conceptually, gravitational interactions are not really different - one could say the Moon is continually in collision with the Earth - but that sounds weird because of the everyday connotation of the word.
With that out of the way, let me address you question. The first point to make is that dark matter, by definition, does not interact electromagnetically. It does not have electric charge - no positives, no negatives. They interact gravitationally, and possibly via the weak nuclear force (as with Weakly Interacting Massive Particle, or WIMPS).
How strongly particles interact determines how often they collide and how quickly they can come into thermodynamic equilibirum. For example, take a hot mug of coffee you've just made in your air conditioned apartment/house. The hot, quickly moving molecules that make up the coffee collide with the slower molecules in the air. In such a collision, the faster moving particles lose some of their kinetic energy and the slower moving particles gain some kinetic energy. Thus the coffee cools and the air warms up (in proporation to the volume of each substance) until the two are in thermal equilibrium, at the same temperature (and your coffee is now cold).
How great the loss/gain is depends on the strength of the interaction (which is electromagnetic in this example). If the electromagnetic interaction were stronger, the coffee would cool quicker, and vice versa.
Suppose you make your coffee and wait one second. The coffee would still be hot, the air cool. You could say that the temperature distribution is clumpy. On the timescale of 1 second, this is a collisionless system - i.e. the collisions were insufficient to bring the coffee-air system into equilibrium in 1 second or less.
Wait an hour, however, and you'll find that the coffee is cold - it has come into equilibirium and the temperature is uniform. On a timescale of one hour, the system is collisional.
So the terms collisional and collisionless are relative to the timescale of observation. For galaxies, the timescale is roughly the age of the universe. If the dark matter particles are collisional on that timescale, the dark matter halos of galaxies will be uniform - in thermal equilibirium. If they are collisionless, they will be clumpy - i.e. there will be different populations of dark matter particles at different 'temperatures' - i.e. different kinematically.
Very interesting article and explanation. I'm glad cfreza posted this... It helps make "dark matter" a bit clearer in my mind.
ReplyDeleteLove the coffee and time analogy....